The present invention relates to a signal processing circuit which is used as an amplifying circuit, a variable filter circuit or the like.
Conventional amplifying circuits using an operational amplifier are constituted such that an AC voltage signal is supplied, for example, to a non-inverting input terminal of this operational amplifier and an AC output voltage signal of the operational amplifier is negative fed back to an inverting input terminal thereof through a resistor and the like. In the case where it is required to make the amplifying circuit of this kind operative as a general amplifying circuit, it is generally difficult to make this amplifying circuit operative at a low power source voltage. In addition, it is necessary to set a negative feedback amount to be large in order to keep the difference between a DC potential at the input terminal of operational amplifier and a DC potential at the output terminal, namely, the DC offset as small as possible. In this case, it is generally required to provide a large capacitance in the amplifier for prevention of the oscillation. When this kind of amplifying circuit is used, it is generally difficult to operate the amplifying circuit at a low power source voltage.